1. Field of the Invention
This invention relates to a method and means of burning biomass fuels together with fossil fuels for the purpose of achieving a net reduction in the quantity of carbon dioxide emitted to the atmosphere by partially replacing the quantity of fossil fuel burned with a fuel that, as it is naturally replenished, absorbs CO2, while simultaneously reducing the quantity of methane and other gases emitted into the atmosphere from biomass landfills.
2. Description of the Prior Art
The worldwide recognition of the contribution of fossil fuel combustion to the increasing atmospheric carbon dioxide levels has prompted the utility industry to seek steps to reduce its emissions. A number of programs have been proposed, some of which are currently under development. These generally consist of methods of removing and collecting the CO2 from the flue gas for deep ground or sea burial. All such steps involve considerable capital and operating costs that would result in significantly increased cost to the consumer. One relatively simple approach that has merit is the substitution of a portion of the fossil fuel in utility and industrial boilers burning coal or oil with waste biomass materials. The amount of fossil fuel that can be replaced with biomass in existing fossil fuel boilers is generally limited to the order of 15% because of equipment design limitations. Such substitution would thereby provide about 10% of the energy produced. Based partially on its high percentage of moisture, biomass fuel heat content is about two thirds that of the fossil fuel it replaces. Nevertheless, it would make a significant contribution to the emission reduction of both CO2 and, as mentioned above, methane, which has a significantly higher effect on global temperature.
Biomass, particularly that which is produced from waste wood and is relatively dry and size reduced, is currently being fed to power plant and industrial boilers along with coal. Biomass-waste fuels are also currently being produced from a wide range of vegetative sources. Since their physical and chemical properties vary widely, depending upon the source and type of processing, the methods used to feed or inject them into the combustion zone of a fossil-fueled boiler are also varied. Biomass materials are also produced as chars from the extraction of usable chemicals by pyrolysis. Some chars can be fully dried, micronized and pre-mixed with finely ground coal particles for simultaneous combustion. Others can be pelletized for separate feeding with only partial drying. In the latter case, in order to avoid localized zones of excessively high biomass fuel concentration, with humidity and temperature variation, it would be desirable to be able to inject and distribute the pellets throughout the cross-section of the fossil fuel combustion. On the other hand, it is desirable to maximize the feed rate at particular locations so as to minimize the number of fuel feeders or injectors needed (requiring costly boiler wall modifications). In order to distribute the injected biomass fuel from such individual injection points, its acceleration and propulsion is desired. It is also desirable that the air needed for combustion of the biomass fuel be mixed with it during injection so that its combustion does not locally reduce the air needed for the burning fossil fuel particles carried in the flame.
Although several patents searched by the applicant appeared to relate to the subject matter, method or means of the present application, detailed examination of them revealed that none appeared to dominate or read on its disclosure, or anticipate its claims, with the possible exception of the applicant's prior patent, “Variable Gas Atomization,” U.S. Pat. No. 4,314, 670, Feb. 9, 1982. Although their concentric annular nozzles are generally similar, their functions, and the details of the exit configurations so required, are distinctly different. With VGA nozzles, the function is to produce a region of atomization within the zone of maximum mass velocity, which occurs at a nozzle throat, by the action of high velocity compressed air flowing on both sides of a liquid sheet flowing at lower velocity. In the present invention, the concentric annular streams are directed so as to contact the solid particles externally to the nozzle exit. Accelerating solid particles that are large in comparison to the droplet sizes desired with atomization, by contact within a nozzle having an exit width sufficient to pass the particles, requires excessive quantities of compressed air. With VGA nozzles, air is typically compressed to pressures higher than 15 psig, which thereby produces sonic throat velocity and throat pressure above ambient, plus turbulence and eddy formation. In the present invention, these effects are contra-indicated in the interest of maximizing the acceleration in generally axial direction. Pressures less than 15 psig are therefore employed. The energy of the high velocity air stream is thereby primarily directed toward particle acceleration. It is noted that U.S. Pat. Nos. 5,178,533 and 4,428,727 employ a series of concentric annular feed channels to deliver fuel and air for the purpose of combustion. In both cases, however, the nozzle exits are provided with means for producing rotation, swirling and eddies. Since swirling the flows upon exit produces centrifugal force, which is also contra-indicated for axially directed acceleration, the above cited patents are not considered to be relevant prior art